CO2 recovery apparatus and CO2 recovery method

ABSTRACT

Provided are a CO 2  absorber that reduces CO 2  contained in flue gas; a regenerator that reduces CO 2  contained in rich solvent absorbing CO 2  to regenerate the rich solvent, so that lean solvent having the CO 2  reduced in the regenerator is reused in the CO 2  absorber; a heat exchanger that allows the rich solvent to exchange heat with the lean solvent; and a controller that controls to extract rich solvent portion that is part of the rich solvent, to allow the rich solvent portion to by pass the heat exchanger, and to be supplied into the top of the regenerator without exchanging heat so as to minimize a sum of an enthalpy that is taken out of the regenerator as CO 2  gas accompanying steam and an enthalpy of the lean solvent after heat exchange with the rich solvent in the heat exchanger.

TECHNICAL FIELD

The present invention relates to a CO₂ recovery unit and a CO₂ recoverymethod that can dramatically reduce the amount of energy used inregenerating a CO₂ absorbent in a CO₂ recovery process.

BACKGROUND ART

It has come to be pointed out that one of the causes of the globalwarming is a greenhouse effect of CO2, and it has became an urgent task,also internationally, to provide a countermeasure for CO2 to protect theglobal environment against the warming. CO2 is generated by any humanactivities combusting fossil fuels, and there are increasing demands forsuppressing CO2 emissions. Along with such an increasing demand,researchers are energetically investigating a method for reducing andrecovering CO2 included in flue gas, to apply in a power plant thatconsumes a large amount of fossil fuels, such as a thermal plant. Insuch a method, flue gas emitted from a steam generator is brought intocontact with an amine-based CO2 absorbent to allow such absorbent toabsorb the CO2, and the recovered CO2 is stored therein without beingreleased into the air. As processes for reducing and recovering CO2 fromthe flue gas using the CO2 absorbent, Japanese Patent ApplicationLaid-open No. H3-193116, for example, brings flue gas into contact withthe CO₂ absorbent in an absorber, heats an absorbent that has absorbedCO₂ in a regenerator, isolates CO₂ as well as regenerates the absorbent,and circulates the absorbent back to the absorber and reuses theabsorbent therein.

As shown in FIG. 5, a conventional CO₂ recovering apparatus 100 asmentioned above includes a flue gas cooler 14, a CO₂ absorber 16, and aregenerator 18. The flue gas cooler 14 cools flue gas 12 containing CO₂emitted from an industrial combustion facility 11, such as a steamgenerator or a gas turbine, with cooling water 13. The CO₂ absorber 16further includes a CO₂ recovering unit 16A. The CO₂ recovering unit 16Abrings the flue gas 12, containing the cooled CO₂, into contact with CO₂absorbent (hereinafter, also referred to as “absorbent”) 15 that absorbsCO₂, to reduce CO₂ in the flue gas 12. The regenerator 18 causes CO₂absorbent (hereinafter, also referred to as “rich solvent”) 17 that hasabsorbed CO₂ to release CO₂ to regenerate the CO₂ absorbent.

In the CO₂ recovering apparatus 100, the regenerated CO₂ absorbent(hereinafter, also referred to as “lean solvent”) 15 having CO₂ reducedin the regenerator 18 is reused in the CO₂ absorber 16 as the CO₂absorbent.

By a CO₂ recovering method using the CO₂ recovering apparatus 100, aflue gas booster fan 20 raises the pressure of the flue gas 12 emittedfrom an industrial combustion facility such as a steam generator or agas turbine and containing CO₂. The flue gas 12 is then sent into theflue gas cooler 14, cooled by way of the cooling water 13, and then sentinto the CO₂ absorber 16.

The CO₂ absorber 16 then brings the flue gas 12 in a counter-currentcontact with the CO₂ absorbent 15 that is based on amine-based solvent,allowing the CO₂ absorbent 15 to absorb the CO₂ contained in the fluegas 12 by way of chemical reaction.

A washing unit 16B, included in the CO₂ absorber 16, brings the flue gashaving CO₂ reduced in the CO₂ recovering unit 16A into a gas-liquidcontact with circulating condensate water 19. The condensate water 19contains the CO₂ absorbent, and is supplied via a nozzle included in awashing unit 16B. In this manner, the CO₂ absorbent 15 that hasaccompanied the flue gas having CO₂ reduced is recovered. Flue gas 12having CO₂ reduced is released out of the system.

A rich solvent pump 22 increases the pressure of the rich solvent thatis the CO₂ absorbent 17 that has absorbed CO₂. Then, a rich/lean solventheat exchanger 23 heats the rich solvent by way of the CO₂ absorbent 15that is lean solvent regenerated by the regenerator 18, and suppliedinto the regenerator 18.

The rich solvent discharged into the regenerator 18 through the topthereof causes an endothermal reaction, thus releasing a majority ofCO₂. The CO₂ absorbent that has released some or a majority of CO₂ inthe regenerator 18 is called semi-lean solvent. By the time thesemi-lean solvent reaches the bottom of the regenerator 18, almost allof the CO₂ is removed, turning the semi-lean solvent into the absorbent15. A regenerating heater 24 then heats the lean solvent by way of steam25, supplying steam inside the regenerator 18.

CO₂ gas 26 a is guided out from the top of the regenerator 18, togetherwith the steam that has been released from the rich solvent andsemi-lean solvent in the regenerator 18. A condenser 27 then condensessteam contained in the CO₂ gas 26, and a separation drum 28 separateswater 26 b from the CO₂ gas 26. The CO₂ gas 26 c is then released out ofthe system, and recovered separately. The recovered CO₂ gas 26 c isinjected into an oilfield using enhanced oil recovery (EOR) method, orstored in an aquifer as a countermeasure for global warming.

The water 26 b separated in the separation drum 28 is pumped up to thetop of the regenerator 18 by way of a condensed-water circulating pump29. The rich/lean solvent heat exchanger 23 cools the regenerated CO₂absorbent (lean solvent) 15 by way of the rich solvent 17. A leansolvent pump 30 then increases the pressure of the lean solvent 15.After being cooled down by a lean solvent cooler 31, the lean solvent 15is supplied into the CO₂ absorber 16.

In FIG. 5, the reference numeral 11 a denotes to a flue for the flue gas12; the reference numeral 11 b denotes to a stack; and the referencenumeral 32 denotes to steam-condensed water. The CO₂ recoveringapparatus may be either added to an existing flue gas source to recoverCO₂, or installed with a flue gas source that is to be newly installed.A door that can be opened and closed is attached on the stack 11 b. Thedoor is closed while the CO₂ recovering apparatus is operating, andopened while the flue gas source is operating but the CO₂ recoveringapparatus is not operating.

As a method for releasing CO₂ gas from the rich solvent 17 that hasabsorbed CO₂ to regenerate the absorbent, steam stripping is used.According to this method, the absorbent is boiled in the regeneratingheater 24 to generate steam. The absorbent is heated thereby in theregenerator 18, and is regenerated by way of a steam-stripping effectthereof.

Disadvantageously, this method has a high heat loss, because steam isreleased out of the steam system by accompanying the CO₂ gas 26 a thatis brought out of the regenerator 18.

In response to this issue, according to a suggestion disclosed inJapanese Patent Application Laid-open No. 55-9079, for example, in aprocess for regenerating a spent aqueous amine absorbent liquidcontaining acid gas impurities such as hydrogen sulfide and carbondioxide by stripping the acid gas impurities from the spent amineabsorption liquid in a regeneration tower, an improvement includes:splitting spent absorbent liquid stream such that at least a portion ofthe spent absorbent stream passes directly to the top of theregeneration tower; continuously measuring the temperature differencebetween the liquid entering and the vapor exiting overhead at the top ofthe regeneration tower; and also continuously measuring the differencein temperature between the combined mixture of the vapor rising from thebottom of the regeneration tower plus the vapor entering at theintermediate point and the liquid entering at the intermediate point,such that the temperature differentials at both the top and intermediatepoints of the regeneration tower are maintained at temperatures rangingfrom 1 degree to 15 degrees Fahrenheit (F) to reduce steam requirements.

CITATION LIST Patent Literature

-   [PATENT LITERATURE 1] Japanese Patent Application Laid-open No.    H3-193116-   [PATENT LITERATURE 2] Japanese Patent Application Laid-open No.    S55-9079

SUMMARY OF INVENTION Technical Problem

However, according to the amine regenerating method disclosed in theJapanese Patent Application Laid-open No. S55-9079, the enthalpy of theentire system is not taken into account. Therefore, it is desirable toreduce steam requirement considering a heat balance of the entiresystem.

The present invention is made in consideration of the above, and anobject of the present invention is to provide a CO₂ recovery unit and aCO₂ recovery method that can dramatically reduce the amount of energyused in regenerating an absorbent by considering an enthalpy of thesystem of CO₂ absorbing apparatus.

Solution to Problem

According to an aspect of the present invention, a CO₂ recovery unit,which includes a CO₂ absorber that brings flue gas containing CO₂ intocontact with a CO₂ absorbent to reduce the CO₂ contained in the fluegas, and a regenerator that reduces CO₂ contained in rich solventabsorbing CO₂ in the CO₂ absorber to regenerate the rich solvent, sothat lean solvent from which the CO₂ is reduced in the regenerator isreused in the CO₂ absorber, includes: a heat exchanger that allows therich solvent to exchange heat with the lean solvent; and a controllerthat controls to extract part of the rich solvent to allow the part tobypass the heat exchanger, and to be supplied into a top of theregenerator without exchanging heat so as to minimize a sum of anenthalpy that is taken out of the regenerator as CO₂ gas accompanyingsteam and an enthalpy that is a result of subtracting an enthalpy of therich solvent before heat exchange in the heat exchanger from an enthalpyof the lean solvent after the heat exchange.

Advantageously, in the CO₂ recovery unit, control is performed toapproximate a temperature difference between the rich solvent and thelean solvent at an entering side of the heat exchanger and a temperaturedifference between the rich solvent and the lean solvent at an exitingside of the heat exchanger.

Advantageously, in the CO₂ recovery unit, the part of the rich solventthat has bypassed the heat exchanger accounts for 3 to 15 percent byweight of the rich solvent.

Advantageously, the CO₂ recovery unit further includes a first heatexchanging unit that allows the part of the rich solvent that hasbypassed the heat exchanger and been guided into the top of theregenerator to exchange heat in an upper section of the regenerator.

Advantageously, the CO₂ recovery unit further includes a second heatexchanging unit that allows the part of the rich solvent that hasbypassed the heat exchanger to exchange heat with the CO₂ gasaccompanying steam extracted from the regenerator.

According to another aspect of the present invention, a CO₂ recoverymethod, for a CO₂ absorber that brings flue gas containing CO₂ intocontact with a CO₂ absorbent to reduce the CO₂ contained in the fluegas, and a regenerator that reduces CO₂ contained in rich solventabsorbing CO₂ in the CO₂ absorber to regenerate the rich solvent, sothat lean solvent from which the CO₂ is reduced in the regenerator isreused in the CO₂ absorber, includes: allowing the rich solvent toexchange heat with the lean solvent; and controlling to extract part ofthe rich solvent to allow the part to bypass the heat exchanger, and tobe supplied into a top of the regenerator without exchanging heat so asto minimize a sum of an enthalpy that is taken out of the regenerator asCO₂ gas accompanying steam and an enthalpy that is a result ofsubtracting an enthalpy of the rich solvent before heat exchange in theheat exchanger from an enthalpy of the lean solvent after the heatexchange.

Advantageously, the CO₂ recovery method further includes allowing thepart of the rich solvent that has bypassed the heat exchanger and beenguided into the top of the regenerator to exchange heat in an uppersection of the regenerator.

Advantageously, the CO₂ recovery method further includes allowing thepart of the rich solvent that has bypassed the heat exchanger toexchange heat with the CO₂ gas accompanying steam extracted from theregenerator.

Advantageous Effects of Invention

According to the present invention, the heat balance of the entiresystem can be taken into account. As a result, the amount of steamsupplied into the reheater can be dramatically reduced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic of a structure of a CO₂ recovery unit according toa first embodiment of the present invention;

FIG. 2A is a schematic of a heat balance of a lean solvent and a richsolvent in a heat exchanger included in the CO₂ recovery unit shown inFIG. 1;

FIG. 2B is a schematic of a test example of the heat balance of the leansolvent and the rich solvent in the heat exchanger of the CO₂ recoveryunit shown in FIG. 1;

FIG. 3A is a schematic of a structure of a CO₂ recovery unit accordingto a second embodiment of the present invention;

FIG. 3B is another schematic of the structure of the CO₂ recovery unitaccording to the second embodiment;

FIG. 4A is a schematic of a structure of a CO₂ recovery unit accordingto a third embodiment of the present invention;

FIG. 4B is another schematic of the structure of the CO₂ recovery unitaccording to the third embodiment; and

FIG. 5 is an exemplary structure of a conventional CO₂ recovery unit.

DESCRIPTION OF EMBODIMENTS

Embodiments of a CO₂ recovery unit according to the present inventionwill now be explained in detail with reference to the drawings. Theembodiments disclosed herein are not intended to limit the scope of thepresent invention in any way.

Example 1

A CO₂ recovery unit according to a first embodiment of the presentinvention will now be explained with reference to FIG. 1.

FIG. 1 is a schematic of a structure of the CO₂ recovery unit accordingto the first embodiment. FIG. 2A is a schematic of a heat balance of alean solvent and a rich solvent in a heat exchanger included in the CO₂recovery unit according to the first embodiment. FIG. 2B is a schematicof a test example of the heat balance of the lean solvent and the richsolvent in the heat exchanger of the CO₂ recovery unit according to thefirst embodiment. In FIG. 1, the same structures as those in the CO₂recovery unit shown in FIG. 5 are given the same reference signs, andredundant explanations thereof are omitted herein. FIG. 1 depicts theCO₂ regenerator 18 included in the CO₂ recovery unit 100 explainedabove.

As shown in FIG. 1, the CO₂ recovery unit according to the firstembodiment includes: a CO₂ absorber (not shown) that brings flue gascontaining CO₂ into contact with CO₂ absorbent to reduce the CO₂contained in the flue gas; a regenerator 18 that reduces CO₂ containedin rich solvent 17 that has absorbed CO₂ in the CO₂ absorber toregenerate the rich solvent 17, so that lean solvent 15 having CO₂reduced in the regenerator 18 is reused in the CO₂ absorber; a rich/leansolvent heat exchanger (hereinafter, “heat exchanger”) 23 that allowsthe rich solvent 17 to exchange heat with the lean solvent 15; and acontroller 40 that controls to extract rich solvent portion 17-2 that ispart of the rich solvent 17, to allow the rich solvent portion 17-2 tobypass the heat exchanger 23, and to be supplied into the top of theregenerator 18 without exchanging heat, so as to minimize the sum(E1+E2) of an enthalpy (E1) that is taken out of the regenerator 18 asthe CO₂ gas 26 a accompanying steam, and an enthalpy (E2) that is theresult of subtracting an enthalpy of the rich solvent 17 beforeexchanging heat in the heat exchanger 23 from an enthalpy of the leansolvent 15 after exchanging heat.

In FIG. 1, the references sign 18A-1 denotes to a first regeneratingunit; the references sign 18A-2 denotes to a second regenerating unit;the references sign 18B denotes to a washing unit; the references sign18C denotes to a demister; the references sign L₁ denotes to a richsolvent supplying line; the references sign L₂ denotes to a lean solventsupplying line; the references sign L₃ denotes to a split rich solventsupplying line; the references sign 41 denotes to an adjusting valveinserted into the split rich solvent supplying line L₃; and thereferences sign F denotes to a flow meter.

According to the first embodiment, the rich solvent portion 17-2 that ispart of the rich solvent 17 is split into the split rich solventsupplying line L₃, supplied into the regenerator 18, stripped in thefirst regenerating unit 18A-1 included in the regenerator 18 by way ofsteam rising from the bottom of the regenerator 18 to release CO₂ gas.The released CO₂ gas is taken out from the top of the regenerator 18 asthe CO₂ gas 26 a accompanying steam.

Because the extracted rich solvent portion 17-2 does not exchange heatin the heat exchanger 23, the temperature of the rich solvent remainslow, maintaining a great heat exchange capacity. This heat exchangecapacity allows the rich solvent portion 17-2 to exchange heat withsteam in the first regenerating unit 18A-1.

As a result, a less amount of the enthalpy (E1) is taken out of theregenerator 18 as the CO₂ gas 26 a accompanying steam, in comparisonwith a scenario without splitting the rich solvent 17.

A major rich solvent portion 17-1 having exchanged heat in the heatexchanger 23 is supplied into an intermittent area of the regenerator18, and stripped in the second regenerating unit 18A-2 in theregenerator 18 by way of steam rising from the bottom of the regenerator18 to release CO₂ gas.

According to the present invention, the enthalpy (E2) of the leansolvent 15 after exchanging heat in the heat exchanger 23 is obtained.The enthalpy (E1) that is taken out as the CO₂ gas 26 a accompanyingsteam is also obtained. Then, an adjusting valve 41 is adjusted toadjust the amount of the rich solvent 17 to be split under aninstruction of the controller 40, so as to minimize the sum of theenthalpy (E2) and the enthalpy (E1).

The enthalpy (E1) that is taken out as the CO₂ gas 26 a accompanyingsteam can be obtained based on a temperature (T₄) and the pressure ofthe CO₂ gas 26 a that is taken out accompanying steam, and the amount ofthe recovered CO₂.

The enthalpy (E2) of the lean solvent after exchanging heat can beobtained based on a difference between a temperature (T₆) of the leansolvent 15 after exchanging heat in the heat exchanger 23 and thetemperature (T₁) of the rich solvent 17 before exchanging heat in theheat exchanger 23, and the flow rate and the pressure of the leansolvent.

A ratio of the rich solvent portion 17-2 to be bypassed differsdepending on a plant facility; however, it is preferable to set theratio approximately between 3 to 15 percent by weight.

This is because, if the ratio is not within this range, the sum of theenthalpies (E1+E2) will not be minimized.

Furthermore, as shown in FIG. 2A, the controller 40 preferably controlsto approximate a temperature difference at an entering side of the heatexchanger 23 [(the temperature of the lean solvent 15 (T₆)—thetemperature of rich the solvent 17 (T₁): ΔT_(a)] and a temperaturedifference at an exiting side of the heat exchanger 23 [(the temperatureof the lean solvent 15 (T₅)—the temperature of the rich solvent 17(T₂)): ΔT_(b)](ΔT_(a)≈ΔT_(b)).

At this time, because the rich solvent absorbs CO₂ gas in the CO₂absorber, the rich solvent becomes greater in volume approximately by 10percent. Therefore, to make the heat exchange performed in the heatexchanger suitable, the temperature differences should be brought to thesame level. In this manner, the efficiency of the heat exchange can bemade ideal.

More specifically, when part of the rich solvent 17 was not split, asshown in FIG. 2B, the temperature difference at the entering side of theheat exchanger 23 was [(the temperature of the lean solvent 15 (T₆: 60.0degrees Celsius)—the temperature of the rich solvent 17 (T₁: 52.0degrees Celsius)) ΔT_(a): 8.0 degrees Celsius], and the temperaturedifference at the exiting side of the heat exchanger 23 was [(thetemperature of the lean solvent 15 (T₅: 109.4 degrees Celsius)—thetemperature of the rich solvent 17 (T₂: 96.8 degrees Celsius)) ΔT_(b):12.6 degrees Celsius], respectively, and these temperature differenceswere not approximate.

In contrast, when part of the rich solvent 17 was split as disclosedherein, the temperature difference at the entering side of the heatexchanger 23 was [(the temperature of the lean solvent 15 (T₆: 62.1degrees Celsius)—the temperature of the rich solvent 17 (T₁: 52.0degrees Celsius)) ΔT_(a): 10.1 degrees Celsius], and the temperaturedifference at the exiting side of the heat exchanger 23 was [(thetemperature of the lean solvent 15 (T₅: 109.4 degrees Celsius)—thetemperature of the rich solvent 17 (T₂: 99.3 degrees Celsius)) ΔT_(b):10.1 degrees Celsius], and the temperature differences were the same.

In this manner, the heat exchange efficiency of the heat exchanger 23can be maximized.

A particular example of an operation performed by the CO₂ recovery unitwill now be explained.

(1) The CO₂ recovery unit is driven to recover CO₂ from the flue gas.

(2) When the circulation of the absorbent reaches a predeterminedstability, a controller unit, not shown, controls to gradually open theadjusting valve 41 so that part of the rich solvent 17 is split to thesplit rich solvent supplying line L₃.

(3) The enthalpy (E1) taken out as the CO₂ gas 26 a accompanying steamis then obtained. The enthalpy (E2) of the lean solvent 15 after heatexchange is also obtained. A controller, not shown, controls to adjustthe adjusting valve 41 so as to minimize the sum of the enthalpy (E1)and the enthalpy (E2).(4) While the operation is kept running, the controller 40 constantlymonitors to keep the sum of the enthalpies to the minimum.

By controlling according to the present invention, the heat balance ofthe entire system is taken into account. As a result, the amount ofsteam supplied into a regenerating heater 24 can be dramaticallyreduced.

Test Example

A test was performed using an amine-based solution as the CO₂ absorbentto recover 29.0 Nm/H (57.0 Kg/H) of carbon dioxide. In this testsetting, according to the conventional method where no part of the richsolvent 17 that is to be supplied into the regenerator 18 was split, thetemperature of the CO₂ gas 26 a that is taken out from the top of theregenerator 18 accompanying steam was 87 degrees Celsius.

On the contrary, as shown in FIG. 1, when the portion 17-2 of the richsolvent 17 was split to minimize the sum (E1+E2) of the enthalpy (E1)taken out of the regenerator 18 as the CO₂ gas 26 a accompanying steam,and the enthalpy (E2) of the lean solvent 15 after exchanging heat withthe rich solvent 17 in the heat exchanger 23 (33.1 kg/H that wasapproximately 5 percent of 681.1 kg/H of the rich solvent 17 was split),the temperature of the CO₂ gas 26 a accompanying steam declined to 60degrees Celsius.

At this time, the heat lost in the lean solvent 15 was 1,310 Kcal/H, andthe heat gained at the top of the regenerator 18 was 4,830 Kcal/H. Theamount of steam corresponding to the energy of 3,520 Kcal/H that is thedifference between these two was reduced.

Therefore, approximately 7.5 percent of 46,800 Kcal/H that is the amountof steam used in the regenerating heater 24 was reduced to improve theheat efficiency of the entire system.

In other words, according to the present invention, the heat of steam tobe taken out from the top of the regenerator can be recovered toincrease the temperature of the rich solvent, as well as to regeneratethe rich solvent. In this manner, less amount of energy is required tosupply steam in the regenerating heater 24 that generates steam forregeneration of the absorbent.

Example 2

A CO₂ recovery unit according to a second embodiment of the presentinvention will now be explained with reference to FIGS. 3A and 3B.

FIGS. 3A and 3B are schematics of structures of the CO₂ recovery unitaccording to the second embodiment. In these drawings, the samestructures as those in the CO₂ recovery unit shown in FIG. 1 are giventhe same references signs, and redundant explanations thereof areomitted herein.

As shown in FIG. 3A, the CO₂ recovery unit according to the secondembodiment includes a first heat exchanging unit 18D that allows therich solvent portion 17-2 bypassed and introduced from the top of theregenerator 18 to exchange heat at the top of the regenerator.

In the first heat exchanging unit 18D, the rich solvent portion 17-2that is temporarily is brought into the regenerator 18 is heated by wayof rising steam, having the temperature thereof (T₇) increased by thetime of being released into the regenerator 18.

Therefore, the amount of steam used in the regenerating heater 24 can befurther reduced, in comparison with the first embodiment, further toimprove the heat efficiency of the entire system.

More Specifically, as shown in the temperature distribution in FIG. 4B,the steam was supplied into the bottom of the regenerator 18 at thetemperature of 140 degrees Celsius (t₀); after the major rich solventportion 17-1 was stripped by way of rising steam to release CO₂ gas inthe second regenerating unit 18A-2, the temperature of the steam was 86degrees Celsius (t₁); and the temperature of the steam after passingthrough the first regenerating unit 18A-1 was 82 degrees Celsius (t₃).The rich solvent portion 17-2 was then temporarily brought into theregenerator 18 at the temperature of 52 degrees Celsius (T₁). After theheat exchange, the temperature of the rich solvent increased to 62degrees Celsius (T₇).

As a result, it has been demonstrated that, when the amount of usedsteam was 120 kg/H, a reduction by 13.4 kg/H was achieved, accountingfor 12 percent of a steam reduction. At this time, the loan solvent waslet out from the regenerator 18 at the temperature of 109.4 degreesCelsius (T₅); after exchanging heat in the heat exchanger 23, thetemperature of the rich solvent was 99.3 degrees Celsius (T₂); and 10percent of the rich solvent portion 17-2 was split.

Example 3

A CO₂ recovery unit according to a third embodiment of the presentinvention will now be explained with reference to FIGS. 4A and 4B.

FIGS. 4A and 4B are schematics of structures of the CO₂ recovery unitaccording to the third embodiment. In these drawings, the samestructures as those in the CO₂ recovery unit shown in FIG. 1 are giventhe same references signs, and redundant explanations thereof areomitted herein.

As shown in FIG. 4A, the CO₂ recovery unit according to the thirdembodiment includes a second heat exchanging unit 18E that allows theCO₂ gas 26 a accompanying steam, taken out from the regenerator 18, toexchange heat with the bypassed rich solvent portion 17-2.

Because the rich solvent portion 17-2 that is before being supplied intothe regenerator 18 exchanges heat with the CO₂ gas 26 a accompanyingsteam that is to be released out, the rich solvent portion 17-2 isheated by steam, having the temperature thereof (T₈) increased, by thetime the rich solvent portion 17-2 is released into the regenerator 18.

Therefore, the amount of steam used in the regenerating heater 24 can befurther reduced, in comparison with the first embodiment, improving theheat efficiency of the entire system further.

More specifically, as shown in the temperature distribution in the FIG.2B, the steam was supplied into the bottom of the regenerator 18 at thetemperature of 140 degrees Celsius (t₀); after the major rich solventportion 17-1 was stripped by way of rising steam to release CO₂ gas inthe second regenerating unit 18A-2, the temperature of the steam was 86degrees Celsius (t₁); and the temperature of the steam after passingthrough the first regenerating unit 18A-1 was 82 degrees Celsius (t₃).The steam was released out of the regenerator 18 at the temperature of80 degrees Celsius (T₄). The temperature of the rich solvent portion17-2 was 52 degrees Celsius (T₁). After the heat exchange performed inthe second heat exchanging unit 18E, the temperature increased to 62degrees Celsius (T₈).

As a result, it has been demonstrated that, when the amount of usedsteam was 120 kg/H, a reduction by 13.4 kg/H was achieved, accountingfor 12 percent of a steam reduction. At this time, the lean solvent waslet out from the regenerator 18 at the temperature of 109.4 degreesCelsius (T₅); after exchanging heat in the heat exchanger 23, thetemperature of the rich solvent was 99.3 degrees Celsius (T₂); and 10percent of the rich solvent portion 17-2 was split.

The invention claimed is:
 1. A CO₂ recovery unit, which includes a CO₂absorber that brings flue gas containing CO₂ into contact with a CO₂absorbent to reduce the CO₂ contained in the flue gas, and a regeneratorthat reduces CO₂ contained in rich solvent absorbing CO₂ in the CO₂absorber to regenerate the rich solvent, so that lean solvent from whichthe CO₂ is reduced in the regenerator is reused in the CO₂ absorber,comprising: a heat exchanger that allows the rich solvent to exchangeheat with the lean solvent; and a controller that controls to extractpart of the rich solvent to allow the part to bypass the heat exchanger,and to be supplied into an upper section of the regenerator withoutexchanging heat so as to minimize a sum of an enthalpy that is taken outof the regenerator as CO₂ gas accompanying steam and an enthalpy that isa result of subtracting an enthalpy of the rich solvent before heatexchange in the heat exchanger from an enthalpy of the lean solventafter the heat exchange, wherein the controller includes: a plurality ofsensors for detecting at least temperatures provided on a rich solventsupplying line and a lean solvent supplying line respectively; a flowmeter provided on a split rich solvent supplying line; and an adjustingvalve provided on the split rich solvent supplying line downstream ofthe flow meter to thereby adjust the amount of the rich solvent to besplit under an instruction of the controller by adjusting the adjustingvalve based on the information obtained by the plurality of sensors andthe flow meter so as to approximate a temperature difference between therich solvent and the lean solvent at an entering side of the heatexchanger and a temperature difference between the rich solvent and thelean solvent at an exiting side of the heat exchanger.
 2. The CO₂recovery unit according to claim 1, wherein the controller controls sothat the part of the rich solvent that has bypassed the heat exchangeraccounts for 3 to 15 percent by weight of the rich solvent.
 3. The CO₂recovery unit according to claim 1, first heat exchanging unit thatallows the part of the rich solvent that has bypassed the heat theregenerator wherein the split rich solvent supplying line is guided intoan upper section of the regenerator and constitutes a first heatexchanging unit by which a rich solvent portion exchanges heat in anupper section of the regenerator, and wherein the split rich solventsupplying line is once out of the generator and again guided into theupper section of the regenerator to release the rich solvent portioninto the regenerator.
 4. The CO₂ recovery unit according to claim 1further comprising a second heat exchanging unit that allows the part ofthe rich solvent that has bypassed the heat exchanger to exchange heatwith the CO₂ gas accompanying steam extracted from the regenerator.